1. Field of the Invention
The present invention relates to a spark plug for use in igniting internal combustion engines.
2. Description of the Related Art
As FIG. 10 shows, a spark plug 300 for firing internal combustion engines such as a gasoline engine for automotive and other applications is used after being fitted on an engine cylinder head SH by means of a fitting thread portion 301a formed on the outer peripheral surface of a metallic shell 301. If the spark plug is thusly fitted, a spark discharge gap g formed between a ground electrode 304 and a center electrode 303 is situated within a combustion chamber BR to serve as a site where an air-fuel mixture is fired. With the recent increase in the performance of engines, mechanisms around the cylinder head have become complicated and more engines are being operated with the spark plug 300 fitted in a deeper position through a plug hole PH made in the cylinder head.
The conventional spark plug has a hexagonal portion formed in the intermediate area of the metallic shell so as to assist in tightening of the spark plug. For fitting this spark plug on the above-described recent type of engines, the inside diameter of the spark plug hole PH must at least be equal to the sum of the outside diameter of the hexagonal portion and the allowance for engagement with a tightening tool such as a socket wrench so as permit its insertion into the plug hole. However, with the increasing complexity of the cylinder head, a smaller space is available around the valve on which the spark plug is to be fitted and this has presented with a demand for minimizing the size of the plug hole PH. For more efficient firing, there has recently been developed a new type of engine that is fitted with more than one spark plug (for example, three) in one cylinder and with such engines, the diameter of the plug hole PH is inevitably very small.
If the plug hole PH is small, the diameter of the tool such as a wrench also has to be reduced and so is the outside diameter of the hexagonal portion of the spark plug which the tool is to engage. However, with the above-described conventional spark plug, the diameter of the insulator has to be smaller than the outside diameter of the hexagonal portion and in practice a very thin (for example, no more than 5 mm in outer diameter) insulator is required. However, thin insulators are prone to be insufficient in mechanical strength and withstand voltage. What is more, when producing the insulator by shaping and sintering a powder of insulating material, it is difficult to prepare a shaped powder by pressing or other forming techniques; in addition, defects such as bends are prone to occur during sintering. These problems combine to deteriorate the process economics.